1. Field of the Invention
The present invention relates to an improvement of a transparent electrode in a group III nitride semiconductor light-emitting device.
The present application is based on Japanese Patent Application No. Hei. 11-181020, which is incorporated herein by reference.
2. Description of the Related Art
The-configuration of a background-art transparent electrode has been described in Unexamined Japanese Patent Publication No. Hei. 9-320984. The background-art transparent electrode has a layered structure with a first layer of a first metal formed on a p-type layer as the uppermost layer of a group III nitride semiconductor light-emitting device, and a second layer of a second metal laminated on the first layer. In the configuration, the first metal (Ni: 25 xc3x85 thick in an example) has a lower ionization potential than that of the second metal (Au: 60 xc3x85 thick in the example). The reason why the first metal lower in ionization potential is formed on the p-type layer is that the first metal high in reactivity is firmly bonded to the p-type layer.
When such a transparent electrode film having a two-layered structure formed on a p-type layer is subjected to a heat treatment, metal elements for forming the respective layers diffuse each other. As a result, distributions of the metal elements are substantially inverted after the heat treatment. That is, the first metal is turned up.
The transparent electrode is coated with a protective film of silicon oxide, or the like. The protective film is formed after the aforementioned heat treatment.
The inventors of the present invention have eagerly investigated the aforementioned transparent electrode film but have been confronted by the following problem.
That is, the laminate of the first and second metals for forming the transparent electrode film was often observed peeling from the p-type layer though firm adhesion due to the first metal was expected. According to the inventors""examination, this was conceived to be for the following reason. The laminate for forming the transparent electrode film was subjected to a chemicals treating process, a cleaning process, a plasma ashing process, etc. before the laminate was subjected to the heat treatment. For example, the laminate for forming the transparent electrode film was formed by a lift-off method using a photo resist formed on the p-type layer. The laminate for forming the transparent electrode film was, however, affected by a releasant solution for releasing the photo resist. Particularly, the releasant solution had influence on the high-reactive first metal to weaken its adhesion to the p-type layer. Because such a photo resist was used also for forming an electrode pad, a releasant solution used therefor had further influence on the first metal.
Other processes such as an acetone ultrasonic cleaning process and a plasma ashing process were assumed to have influence on the first metal which was low in ion potential and, accordingly, high in reactivity.